The present invention relates to a new and improved construction of a passive infrared detector for determining the presence of a body, for instance an intruder or unauthorized person in a monitored area or room.
In its more specific aspects, the present invention concerns a new and improved construction of a passive infrared detector for determining the presence of a body, typically a human being, possessing a temperature deviating from the ambient temperature. The passive infrared detector comprises at least one sensor element for generating an electrical signal as a function of infrared radiation impinging thereat, at least one optical element or system serving for focussing onto the sensor element the infrared radiation emitted by the body, as well as an evaluation circuit serving for monitoring the electrical signals outputted by the sensor element.
It is known to use infrared detectors in monitoring equipment for determining the presence of intruders in rooms or areas which are to be supervised. These infrared detectors, so-called passive-IR-detectors, are responsive to the infrared radiation emitted by a body, especially by human beings. A drawback of such infrared detectors and the presently employed wide-band sensitive sensor elements, such as pyroelectric crystals or polymers, bolometers or thermoelements, resides in the fact that these elements are responsive to electromagnetic radiation throughout the entire wavelength range. Consequently, there are also generated signals, which although predicated upon infrared radiation, are not generated by any intruders. Such false alarms must be prevented to the utmost extent possible in any good intrusion monitoring system.
Therefore, attempts have repeatedly been made to find possibilities which safeguard passive infrared detectors against issuing false alarms. In German Pat. No. 2,103,909, published Nov. 25, 1976, there is for instance disclosed such type of monitoring apparatus, wherein an adequate coverage of a particularly large total region or area is obtained by means of only one feeler element or sensor which only then delivers a clear differentiable output signal whenever an intruder moves across the boundary of the covered or monitored region. This is achieved in that a number of reflecting surfaces are arranged such that these reflecting surfaces direct the infrared radiation emanating from a number of mutually separate fields of view upon the feeler element.
To avoid false alarms by electromagnetic radiation which is within a wavelength range which does not correspond to that of a black body (intruder) in a temperature range of 0.degree. C. to 40.degree. C., the radiation inlet window of the infrared detector is covered with an optical filter having a throughpass range of 4 to 20 .mu.m. Consequently, there is especially blocked visible light. Furthermore, the signal delivered by the feeler or sensor element is amplified by an alternating-current amplifier which is structured such that there are only amplified signals in the frequency range corresponding to the passage of an intruder through the different zones of the region or area to be monitored. This frequency range preferably lies in the order of between 0.1 Hz and 10 Hz.
To detect the presence of intruders in a room or area to be monitored it is necessary to monitor the entire room or area, i.e. both the near region and also the far region, in order to preclude the need for mounting a multiplicity of detectors. In U.S. Pat. No. 3,480,775, granted Nov. 25, 1969, there is disclosed a passive infrared detector, wherein the infrared radiation impinges upon the infrared sensor by means of a substantially cylindrical-shaped fine grid which is arranged about the infrared sensor. Consequently, there is possible an omnidirectional monitoring and a differentiation between background radiation, since a moving body emitting infrared radiation generates an electrical alternating-current signal. To differentiate a moving body emitting infrared radiation from background radiation, the room or area to be monitored is generally divided into fan-like monitoring regions or zones, for instance by means of a zone optical system.
In U.S. Pat. No. 3,829,693, granted Aug. 13, 1974, there is disclosed a passive infrared intrusion detector where thermoelements or thermistors or pyroelectric detectors, serving as the infrared sensors, are arranged in different columns in such a manner that elements of the same column possess the same polarity, yet differ from the polarity of the neighboring columns, so that a moving body emitting infrared radiation generates an alternating-current signal. The infrared detector is provided with two optical systems laving different focal lengths in order to focus the infrared radiation upon the infrared sensor, and wherein, for instance, a mirror arranged behind the infrared detector, and having a larger focal length than a germanium lens arranged forwardly of the infrared detector, which monitors the near region, serves for increasing the far sensitivity.
In European Patent Application No. 25,983, published Apr. 1, 1981, there is disclosed an infrared motion detector or alarm system wherein for the purpose of reducing the sensitivity in relation to electromagnetic radiation which penetrates through glass, an optical filter located forwardly of the inlet of the infrared detector is connected with a heat sink in the form of a solid metal body. This arrangement, while affording a suppression of the secondary infrared radiation source, cannot however prevent the giving of false alarms by heat turbulence in rooms, since such turbulence emits radiation in a range of 4-20 .mu.m, in other words radiation corresponding to that of intruders.
There are also used in passive infrared detectors differential elements, i.e. the spatial or room zones are imaged upon two closely neighboring sensor elements, for instance two electrodes mounted at the same element, and which are then operatively connected with a differential amplifier. Such type of sensor arrangement has been disclosed, for instance, in U.S. Pat. No. 3,839,640, granted Oct. 1, 1974. In the near region the zones imaged at the individual elements are overlapping, i.e. turbulence generates at both elements the same electrical signals, in other words, the differential amplifier output remains unaffected. By means of such differential elements it is possible to successfully suppress turbulence related which signals are only disturbing if such arise in the near region of the detector. But unfortunately, however, there is also markedly reduced the sensitivity to objects moving in the near range or they cannot be detected at all, quite similar to the case when there occurs turbulence. In other words, intruders which are located close to the detector cannot be detected. Equally, acts of sabotage, such as covering the detector, overspraying the same with a coating material and similar sabotage acts, also cannot be detected.
In European Patent Application No. 23,354, published Feb. 4, 1981, there is disclosed a pyrodetector containing two pyroelectric sensors. One of these pyroelectric sensors is located at the focal point of a hollow mirror or reflector which reflects infrared radiation, whereas the other pyroelectric sensor is located outside of the focal point and serves for the compensation of the infrared radiation which particularly emanates from the cover member.
Room or area monitoring systems operating by means of ultrasound are described in U.S. Pat. No. 4,382,291, granted May 3, 1983, and U.S. Pat. No. 4,499,564, granted Feb. 12, 1985. The room or area monitoring system described in U.S. Pat. No. 4,499,564 like the room or area monitoring described in U.S. Pat. No. 4,382,291, forms a plurality of reference patterns in the normal state of the room or area to be monitored and computes and stores a statistic evaluation of the reference patterns based on the mean value and the standard deviation at predetermined sampling points. An actual monitoring pattern is compared with the statistic evaluation of the reference pattern at the same sampling points. An alarm is generated when the actual monitoring pattern at one of the sampling points deviates from the mean value determined for the reference patterns by more than the standard deviation also computed from the reference patterns.
The room or area monitoring system according to U.S. Pat. No. 4,382,291 does not include measures for suppressing faulty alarms. The room or area monitoring system according to U.S. Pat. No. 4,499,564 attempts to suppress faulty alarms which are due to predetermined noise sources like, for example, a telephone bell or the bell of a fire alarm. Other noise sources like, for example, radio or television loudspeakers, heat turbulences due to heaters, insolation or wind movements, cannot be suppressed so that faulty alarms still occur. The aforementioned systems do not offer a solution for the problem of protection against sabotage, i.e. the covering of the ultrasound sensors by means of adhesive tapes or sprayed-on paints.
While the different known measures for suppressing false alarms are indeed effective, nonetheless they only encompass and deal with a part of the problem of detectors issuing false alarms, and in particular the sabotage problem. This last-mentioned problem is particularly concerned with the intentional covering of the inlet window of the detector with an object, for instance a hat or board, or by spraying-on a transparent lacquer or varnish which absorbs the infrared radiation in the wavelength range of 4-20 .mu.m which is required for the detection of intruders. In this way it is possible to render the detector so-to-speak "blind", and thus, intruders which unlawfully enter the monitored region or room no longer can be detected.
A further problem which has not yet been described in the relevant publications resides in the fact that present day passive infrared detectors must possess a signal-to-noise ratio (S/N) of approximately 10 before the detector can give an alarm. Ihis signal-to-noise ratio had to be selected to be so large, in order that there could be reduced the number of false alarms which were caused by the noise of the detector. A signal-tc-noise ratio S/N of approximately 10 is, however, associated with quite appreciable drawbacks as concerns the detection of intruders, since the signal produced by the object is proportional to the temperature difference between the object and the background. Additionally, the signal of the presently employed pyroelectric sensor elements is proportional to the speed with which the object moves through the room or area to be monitored. Because of this high signal-to-noise ratio which is needed for suppressing false alarms it is difficult to detect intruders who move very slowly and/or who reduce the temperature difference between themselves and the surroundings, for instance by wearing suitable clothes.